A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite their U-like curvature, a dividing plate which is arranged with the tube sheet to form a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle, a primary fluid inlet nozzle in fluid communication with the inlet header, and a primary fluid outlet nozzle in fluid communication with the outlet header. The steam generator also comprises a wrapper sheet disposed between the tube bundle and the shell to form an annular chamber with the internal wall of the shell, and a feedwater ring disposed above the U-line curvature end of the tube bundle. The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid flows through the primary fluid inlet header, through the tubes of the bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator through the feedwater ring. The feedwater is directed down the annular chamber adjacent to the shell until the tube sheet near the bottom of the annular chamber causes the feedwater to reverse direction, passing in heat transfer relationship with the outside of the U-shaped tubes of the bundle and up through the inside of the wrapper. While the feedwater is circulating in heat transfer relationship with the tubes of the bundle, heat is transferred from the primary fluid in the tubes to the feedwater over the outside of the tubes, causing some predetermined portion of the feedwater to be converted to steam. The steam then rises and is circulated through typical electrical generating equipment producing electrical energy in a manner well known in the art.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the walls of the U-shaped tubes which may be constructed from Inconel, the U-tube walls form part of the primary boundary for isolating these radioactive particles. It is, therefore, important that the U-tubes be maintained defect-free so that no ruptures will occur in the U-tubes. However, experience has shown that under certain conditions the U-tubes may develop leaks therein which allow radioactive particles to contaminate the feedwater, a highly undesirable accident.
The subject matter of U.S. application Ser. No. 423,953 filed Sept. 27, 1982 by Creek, et al. discloses a transporter for inspecting defective tubes and sludge accumulation requiring removal, together with instruments with which sludge removal is initiated. However, a transport structure is needed with which to inspect the annular space between the tube bundle and the inside wall of the generator for objects requiring removal.
During repair and general inspection of steam generators, internal objects sometimes are accidentally dropped. These lost objects are likely to find their way to the annular space at the top of the tube sheet between the tube bundle and the vessel wall. Severe damage to tubes has resulted in some generators as a result of these lost objects impacting against the tubes during operation. Motion of these objects is caused by the recirculating fluid within the generator. There is a need for inspecting the annular region in nuclear steam generators to locate and remove foreign objects.